Companding circuits that include a compressor circuit and an expander circuit may be used to increase the useable dynamic range of a signal that passes through a modulated space. In audio applications, this is done by first compressing the dynamic range of the information signal prior to modulation, and then expanding the dynamic range of the information signal after demodulation. Referring to FIGS. 1A and 1B, general, schematic representations of a typical compression circuit 10 and a typical expansion circuit 12, respectively, are shown.
In the circuit shown in FIG. 1A, a rectifier and filter element 14 is used to detect the amplitude of an input signal at terminal 16. The amplitude information is utilized to control the resistance of a variable resistance element 18 that is provided in the feedback loop of an operational amplifier 20. This circuitry is arranged so that relatively higher amplitude information generally reduces the resistance of the variable resistance element 18, while relatively lower amplitude information generally increases the resistance of the variable resistance element 20. This has the effect, for example, of reducing the gain of amplifier 20 for higher level signals at terminal 16, and of increasing the gain of amplifier 20 for lower level signals at terminal 16. Signal processing is continuously done across the frequency spectrum for signals of low amplitudes to signals of high amplitudes.
FIG. 1B is a general, schematic diagram of a typical expander circuit 12 that is used to expand the dynamic range of the signal at terminal 22. Circuit 12 includes generally the same circuit components that form the compression circuit shown in FIG. 1A, such components including a variable resistance element 24, a rectifier and filter element 26, an operational amplifier 28, and a feedback resistor 30. However, the circuit components shown in FIG. 1B are rearranged as shown so that the gain of the operational amplifier 28 increases as the amplitude of the signal at terminal 22 increases, and so that the gain of the operational amplifier 28 decreases as the amplitude of the signal at terminal 22 decreases.
The problems associated with such compandor circuitry are largely due to the time constant of the integrating filter of the rectifier that forms a portion of the rectifier and filter components 14 and 26 (FIGS. 1A and 1B). If the time constant is made relatively large, then amplitude modulation of the higher frequency components of the noise by lower frequency components of the signal can be heard by a user. This is commonly referred to as “breathing,” which is undesirable, especially in high-end audio applications.
Signals with a quick rise time often are distorted by typical compandor circuits because, for example, the compressor circuit portion of the compandor circuit may not be able to react fast enough to keep the signal within the linear range of the modulated space. If the time constant is short, “breathing” goes away, but lower frequency signals become distorted due to rectifier ripple. Both of the above-referenced problems are particularly evident in wide band audio implementations such as, for example, wireless microphone applications.
Distortion problems can be reduced by establishing separate attack and release time constants for the rectifiers that are used in the compression and expansion circuits. However, there is still a serious compromise in performance that must be made for wide band signals present in high-end audio applications.
Various specific compander circuits are known. See, for example, U.S. Pat. No. 4,353,035 that discloses a circuit for compression or expansion of an electrical signal. This patent states that a two-band compander pre-emphasis is carried out during compression and de-emphasis during expansion in the lower frequency range. This patent states that noise suppression is improved by the pre-emphasis and de-emphasis operations. The content of U.S. Pat. No. 4,353,035 is incorporated by reference into this application as if fully set forth herein.
In another application, U.S. Pat. No. 4,449,106 discloses a noise reducing apparatus that includes a circuit that processes signals in a plurality of frequency bands. This patent states that the mid and high frequencies are intensified when the signal level is low. This patent also states that the signal level versus noise level ratio in the mid and high frequency ranges is increased with respect to the noise introduced in the transmission path. The content of U.S. Pat. No. 4,449,106 is incorporated by reference into this application as if fully set forth herein.
In yet another application, U.S. Pat. No. 5,832,097 discloses a multi-channel synchronous companding system for hearing aids. This patent states that an input signal is directed through a 2:1 compressor, and then is directed through a band splitting filter to divide the signal into a desired number of frequency bands. This patent further states that the divided signals are then passed through expander/compressors to provide selected expansion/compression of each frequency band as a function of the user's hearing impairment. The content of U.S. Pat. No. 5,832,097 is incorporated by reference into this application as if fully set forth herein.
The above-described circuits are suitable for their intended purposes. However, such circuits may not be suitable for a number of applications such as, for example, high end wireless microphone applications. In such applications, a premium is placed on the quality of the audio reproduction. Also, a premium is placed on the ability to manufacture such microphones with reduced cost that allows the manufacturer's profit margin to be maximized. Furthermore, battery life is a concern in the transmitter portion of typical wireless microphones because the presence of a greater number of active elements increases current drain and correspondingly decreases battery life.